Mechanical tolerances, temperature-related and pressure-related changes in length, the effects of aging, especially in a PMA (Piezoelectric Multilayer Actuator), referred to below as a “piezoactuator” in a fluid valve, have a direct effect on the opening stroke of the fluid valve connected to the piezoactuator and thereby on the metered quantity. Conventional methods used to compensate for temperature-related changes in length to the piezoactuator based on suitable combinations of materials however present serious stability and manufacturing problems.
The elongation ratio of the piezoactuator which can be achieved by the inverse piezoelectric effect in high-performance ceramics as a result of the application of a maximum field strength of appr. 2 KV/mm permissible for continuous operation only amounts to 1.2-1.4 promille (that is 1.2-1.4 pm elongation per 1 mm length of the piezoactuator). For a typical length of piezoactuator of appr. 40 mm and a piezolayer spacing of 80 μm at 160V applied voltage, the inverse piezoelectric effect produces an elongation of maximum 56 μm. Thus if there is only a minimal relative deviation in the effective coefficient of thermal expansion of appr. 1*10−6 1/K over the length of the piezoactuator of 40 mm between the piezoactuator and the housing in which the piezoactuator is installed, in the range of temperatures of 40° C. to 140° C. relevant to automotive technology, this leads to a deviation of the reference surfaces relevant for valve operation of −2.4 μm to +4.8 μm or in total to 7.2 μm, and relative to the elongation of the piezoactuator to a variation bandwidth of up to 13%.
In addition the complex process steps in manufacturing, starting with the construction of the piezoactuator ceramics through to the polarization process, lead to component tolerances which make it difficult to keep the temperature expansion of the piezoactuator within a sufficiently narrow field of tolerances.
Since the piezoactuator is a component with a domain structure and hysteresis the temperature expansion coefficient is heavily dependent on the polarization state and the previous history of mechanical and electrical stress on the piezoactuator. The dependency of the length of the piezoactuator on temperature is non-linear. The coefficient of thermal expansion can assume values for the same piezoactuator ranging from −5*10−6 1/K up to +7*10−6 1/K [1].
The positive change in length caused by the electrical charging of the piezoactuator is used in current common rail diesel injectors to close a sealing element. For reasons of tolerance in this case a “thermal gap”, that is a safety margin of typically 3-5 pm between the freely-moveable end of a piezoelectric actuator unit (PAU) which is embodied as a plunger or which is rigidly mechanically coupled to a plunger and the sealing element is provided. The PAU consists of an upper end cap which is mechanically rigidly supported and which contains at least one whole through which the electric connections of the piezoactuator can be routed outwards, a lower end cap which is embodied as a plunger or which is mechanically rigidly coupled to a plunger, the piezoactuator and a tubular spring into which the piezoactuator is welded under a pre-tensioning pressure of appr. 600N-800N between the two end caps. It is not possible to ideally strike a thermal balance between the actuator housing and the PAU. The safety margin is used, in the event of a greater thermal expansion of the PAU relative to the actuator housing, so that the sealing element is opened and there is continuous leakage through the servo valve as a result. However the fluctuations in the PMA temperature coefficients make it clear that even such a margin is not always sufficient.
Directly after the injector is switched off (the motor vehicle or engine is switched off) units of the injector are at high temperature. The associated thermal expansion of the piezoactuator relative to the housing which cannot be perfectly tuned can lead to the thermal margin being exceeded and the sealing element being opened despite lack of piezo activation, particularly if in the off state no opposing force F0 caused by the fluid pressure can operate on the sealing element any longer. The sealing element thus remains open in the switched-off state of the engine.
The fluid pressure which is exerted on the sealing element from the other direction can however subsequently in the switched-on state of the injector reach a pressure of up to 2000 bar and give rise to forces or opposing forces of up to 600 N. During injector operation these forces ensure a defined closure of the sealing element despite an overextension of the actuator. An internal high-pressure pump in the motor vehicle, when another attempt is made to start the engine, and thereby the injector, is however no longer in a position if the injector is still hot, to build up the necessary pressure in order to close the sealing element so that this leads to malfunctions of the injector.
An actuator unit A in accordance with the prior art is shown in FIG. 1. It consists of a housing 1, a piezoactuator 2 with a tubular spring 8, a first and a second end cap 3, 7, with the first end cap 3 being provided with a plunger 4. The piezoactuator 2 is welded into the tubular spring 8 under a pre-tensioning pressure of appr. 600 to 800 N in order to avoid damaging tensile stresses during operation. A membrane 5, typically made of metal, enables a seal to be provided between the piezoactuator and fuel. The second end cap 7 is supported against the housing 1 whereas the first end cap 3 on activation presses together with the plunger 4 against the sealing element 6 of the seating valve 12. In the zero-pressure state the sealing element 6 implemented as a ball, is held in the seat 12 with the aid of a weak return spring (not shown) at the pressure of approximately 5N. In the normal state (no activation of the piezoactuator) there is a safety margin between the sealing element 6 and the piston 4 of typically 3 to 5 μm.
In this layout a stronger thermal expansion of the piezoactuator 2, because of its attachment via the end cap 7 to the fixed end of the housing 1 leads to an extension of the piezoactuator in the direction of the valve seat 12.
It should however be pointed out that thermal changes are not short term processes in the range of below 10 ms but take seconds or minutes to occur. This type of slow expansion of the actuator 2 can however be balanced out by a hydraulic compensation element X, as shown in FIG. 1a. Such a hydraulic compensation element X is preferably seated between the end cap 7 of the actuator 2 and the other end of the housing 1 and is attached to the housing. When this type of hydraulic compensation element is used the thermal expansion of the actuator now occurs in the direction of the end cap 7 and does not absolutely lead to a change in the distance between the sealing element 6 and the plunger 4 and thus also does not lead to permanent leakages.
The hydraulic compensation element X however exhibits a stiffness comparable with a rigid body when force is applied to it for short periods, in which case despite this stiffness the hydraulic compensation element or a component of the hydraulic compensation element which is connected indirectly or directly to the piezoactuator gives way by a negligible amount. However these distances, which are in themselves negligible, add up with multiple activation of the piezoactuator so that the hydraulic compensation aliment or the component of the hydraulic compensation element is shifted upwards by the maximum deflection of the piezoactuator and thereby the gap between the piston 4 and the sealing element 6 is enlarged such that the piston no longer reaches this sealing element on repeated actuation of the piezoactuator. Opening the sealing element 6 is no longer possible in this case.